Perpendicular magnetic anisotropy in as-deposited CoFeB/MgO thin films

TL;DR

This study demonstrates that CoFeB/MgO thin films can exhibit perpendicular magnetic anisotropy directly after deposition without post-annealing, with oxygen diffusion control being crucial for PMA and device stability.

Contribution

It reveals that PMA in CoFeB/MgO films can be achieved during deposition by controlling MgO thickness and oxygen diffusion, challenging the previous reliance on post-annealing.

Findings

PMA can be established on various buffer layers without post-annealing.

Precise MgO thickness controls oxygen diffusion and PMA.

Post-annealing enhances PMA by modulating oxygen, not crystallinity.

Abstract

Fabrication of perpendicularly magnetized ferromagnetic films on various buffer layers, especially on numerous newly discovered spin-orbit torque (SOT) materials to construct energy-efficient spin-orbitronic devices, is a long-standing challenge. Even for the widely used CoFeB/MgO structures, perpendicular magnetic anisotropy (PMA) can only be established on limited buffer layers through post-annealing above 300 {\deg}C. Here, we report that the PMA of CoFeB/MgO films can be established reliably on various buffer layers in the absence of post-annealing. Further results show that precise control of MgO thickness, which determines oxygen diffusion in the underneath CoFeB layer, is the key to obtaining the as-deposited PMA. Interestingly, contrary to previous understanding, post-annealing does not influence the well-established as-deposited PMA significantly but indeed enhances unsaturated PMA with a thick MgO layer by modulating oxygen distributions, rather than crystallinity or Co- and Fe-O bonding. Moreover, our results indicate that oxygen diffusion also plays a critical role in the PMA degradation at high temperature. These results provide a practical approach to build spin-orbitronic devices based on various high-efficient SOT materials.